Aluminum-Nitrogen Rings and Cages with Organic Handles: A Theoretical Study Cong Trinh, † Alexey Y. Timoshkin,* ,† and Gernot Frenking ‡ Inorganic Chemistry Group, Department of Chemistry, St. Petersburg State UniVersity, UniVersity Pr. 26, Old Peterhof 198504, Russia, and Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany ReceiVed: July 4, 2008; ReVised Manuscript ReceiVed: January 2, 2009 The structures and stability of monomeric and oligomeric compounds resulting from the elimination of 2 mol of HBr from the AlBr 3 en complex have been studied theoretically at the B3LYP/LANL2DZ(d,p) level of theory. Two series of the ring and cage oligomer compounds were considered: imino [AlBrNCH 2 CH 2 NH 2 ] n , n ) 1-4, 6 (I n ), and isomeric amido [AlBrNHCH 2 CH 2 NH] n , n ) 1-3(A′ n ), species. In all cases the amido isomers are 95-295 kJ mol -1 more stable compared to their imino counterpairs. Intramolecular donor-acceptor interactions play an important role in the stabilization of the structure. Possible pathways of the polymerization have also been explored. Although the generation of the oligomers via HBr elimination in the gas phase is predicted to be thermodynamically unfavorable, operation of such reactions in the condensed phase is feasible. Substitution of bromine by hydrogen makes H 2 elimination reactions thermodynamically favorable. This opens a perspective for the synthesis of novel ladder-type oligomers with a continuous (AlN) n core supported by C 2 H 4 balustrade and imino cages with C 2 H 4 handles. 1. Introduction Group 13-15 binary compounds serve as UV photodetectors, light-emitting diodes, solar cell elements, advanced ceramic materials, and chemical sensors. 1 Chemical vapor deposition (CVD) 2,3 is one of the leading methods for the production of group 13-15 materials. In recent years, fabrication of one-, two-, and three-dimensional group 13-15 nanoarchitectures as a result of chemical reactions has attracted the attention of chemists. Examples on the growth of the group 13-15 nanostructures include controlled gas-phase growth of one-dimensional AlN structures in the aluminum-ammonia system. 4 While the exact mechanism of the process is not fully understood, it is believed that polymeric (oligomeric) species may play an important role. Very recently, pyrolysis of the donor-acceptor-stabilized trisaminoalane Me 3 N-Al[N(H)Dipp] 3 (Dipp ) 2,6- i PrC 6 H 3 ) yielded aluminum-nitrogen oligomers which were converted to AlN in an ammonia atmosphere. 5 Conversion of organome- tallic imino cages to AlN was reported by Koyama et al. 6 and highlighted in a review by Mori and Sugahara. 7 Mixed poly- (alkyliminoalane)s [HAlNR] n (R ) allyl, ethyl) have been synthesized and successfully converted into AlN. 8 It is argued that cross-linking reactions play a role in this conversion. 8 Possible pathways of formation of 2D AlN structures were considered in detail by Himmel 9 using theoretical methods. Recently, donor-acceptor (DA) complexes of group 13 metal trihalides MX 3 with N-containing bidentate donors attracted our attention as potential precursors for the synthesis of group 13 composite nitrides. 10,11 Systematic theoretical studies of the dissociation and HX elimination reactions from complexes of MX 3 (M ) Al,Ga; X ) Cl, Br, I) with ethylenediamine (en) were performed. 12 We experimentally observed that on heating M 2 X 6 en complexes (M ) Al,Ga; X ) Cl, Br, I) eliminate HX and form associates with high molecular weight (600-900 amu). 11 For the Al 2 Br 6 en system, for which in the gas phase the formation of high mass species with high molecular weight was found the most abundant, we also reported a theoretical study of the structures and stabilities of source DA complexes and products resulting from elimination of 1 mol of HBr. 11 In the present paper, we systematically explore with theoretical methods the structures and stability of imino species resulting from elimination of 2 mol of HBr from the AlBr 3 en complex. Such compounds and their derivatives are potential precursors for the controlled synthesis of AlN. 13 Since the AlBr 2 NHC 2 H 4 NH 2 ( ring A) structure was previously identified as one of the key decomposition products, 11 in the present work we considered the ring A compound as a starting point for the next HBr elimination and subsequent oligomerization reactions (Scheme 1). Following the notations from our previous papers, 11,12 in this paper I indicates the monomeric imino compound AlBrNCH 2 CH 2 NH 2 and its respective oligomers with oligo- merization degree n are denoted I n . The isomeric amido structure AlBrNHCH 2 CH 2 NH is denoted as A′ and respective oligomers are denoted as A′ n . Several isomers for each compound have been considered which are distinguished by subscript symbols a, b, etc. 2. Computational Details The Gaussian03 program package 14 was used throughout. The geometries were fully optimized using density functional theory (DFT) with the B3LYP 15,16 hybrid functional in conjunction with the effective core potential basis set LANL2DZ 17 augmented by d and p polarization functions as described in ref 18. The previous study of complexes of group 13 metal halides with ammonia at B3LYP/LANL2DZ(d,p) showed satisfactory agree- ment with high-temperature experimental data. 18 As confirmed by analytical evaluation of the second derivatives, all structures reported here are minima on the potential energy surface. * To whom correspondence should be addressed. Fax: +7(812)428-6939. E-mail: alextim@AT11692.spb.edu. † St. Petersburg State University. ‡ Philipps-Universita¨t Marburg. J. Phys. Chem. A 2009, 113, 3420–3426 3420 10.1021/jp805904e CCC: $40.75  2009 American Chemical Society Published on Web 03/11/2009